They were made to kill all sentient life(flood doesn't count) with in three radii of the universe with sufficient biomass to become a flood host.
Yeah, I don't get that. If the Forerunners could make a weapon that only targets specific entities, WHY THE HELL DIDN'T THEY JUST MAKE IT KILL THE FLOOD?!?
Killing everything is hardly what I call specific, however, I think the main point is that any surviving flood would starve off. In the Halo origins short the flood is seen being distroyed as well, however, its cannon is dispuitable.
The Forerunners can only make weapons against sentient beings flood arent sentient
"[...] Halo doesn't kill Flood, it kills their food. Humans, Covenant, whatever. [...] The only way to stop the Flood is to starve them to death. And that's exactly what Halo is designed to do: Wipe the galaxy clean of all sentient life. You don't believe me? Ask him !"